Where is this CGI script? Is this your own server or do you have a service provider? I know that you can configure servers and even devices(routers, etc) to reject and redirect people from certain IP addresses and only accept a range. Maybe it's that. You might want to talk to whoever runs the server. Also make sure that your service provider (if you're using one) doesn't appened additional source code at the top of your page. For example, in this case, it can check subdomains. I doubt that's the case, but you never know. Make sure you don't have a virus too. There could be a virus that corrupts Internet Explorer so it has problems creating cookies(could become corrupt). If you're having trouble creating cookies, and your login script relies on them, then as a default it could send you to yahoo.com.